This invention relates to a tact switch and its movable contact piece, and particularly to a tact switch in which the flipping-over or reversing action of the cup-shaped curved portion of the movable contact piece will cause the contact section disposed around the periphery of the cup-shaped portion to come into contact with a plurality of fixed contact plates to establish electrical conduction between the fixed contact plates, and the movable contact piece.
U.S. patent application Ser. No. 09/414,067 (filed on Oct. 7, 1999) assigned to the same assignee as the present application proposes a tact switch of the type described herein having external appearances as shown in FIGS. 1A, 1B and 1C. This tact switch comprises a housing, a top lid, an actuator member and a movable contact piece. FIG. 2 is an exploded, perspective view of the tact switch. The construction of the various parts of the tact switch will first be described with reference to FIG. 2.
The housing 11 is made from a synthetic resin material in the form of a box having an open top. Exposed within the box are the inner end portions of four insert-molded fixed contact plates 13 one at each of the four corners of the bottom surface of the box surrounded by side walls 12. Formed in the center of the bottom surface is a circular pedestal 14 having a stud-like protrusion 15 upstanding therefrom. The other end portion of each of the fixed contact plates 13 extends out of the housing 11 and terminates in a terminal 13a.
One pair of the side walls 12 defining major sides of the housing 11 has notches 16 and 17, respectively cut down from the top edges of the side walls in the middle thereof. One 16 of the notches further has a lower recess 18 formed in the center thereof. The bottom surface of the recess 18 is arranged to lie at the same elevation as the bottom surface of the notch 17 in the other side wall 12.
On the other hand, the other pair of the side walls 12 defining minor sides of the housing 11 has notches 19 cut down from the top edges of the side walls and diverging or increasing in width downwardly toward the bottom surface as shown. Each of the notches 19 has one vertically extending inner side surface 19a and the opposed downwardly outwardly inclined or tapered inner side surface 19b.
A stepped portion or ledge 21 extends from the vertical side surface 19a of one of the notches 19 along the interior of that side wall 12 in which the notch 16 is formed to the vertical side surface 19a of the other notch 19, with the bottom surface of the stepped portion 21 being flush with the bottom surfaces of the notches 19 and somewhat lower than the bottom surface of the recess 18.
The side walls 12 in which the notches 16, 17 are formed, respectively have engagement lugs 22 protruding from the outer surfaces thereof at the upper corners, while the other side walls 12 in which the notches 19 are formed have engagement recesses 23 formed in the outer surfaces thereof in the center adjacent the top edges.
The top lid 24 adapted to be mounted on the top of the housing 11 to close the open top of the housing 11 is made of metal and comprise a rectangular top plate 25 having engagement flaps 26 extending and folded down from the opposed minor sides thereof and L-shaped latch tabs 27 extending from the opposed major sides adjacent opposite ends thereof.
The movable contact piece 28 is made in the shape of a cup having small marginal flat portions from a spring material such as a phosphor bronze plate. In the example illustrated, it is made from a generally rectangular sheet so as to define a curved section 29 with longitudinal opposite end portions of the sheet left. The curved section 29 is formed in the center with a positioning hole 31. In addition, the movable contact piece 28 has a notch 32 formed in each of the opposite minor sides in the middle thereof to define contact portions 33 at the opposite ends of the minor side.
The actuator member 34 for press-actuating the movable contact piece 28 is made of a synthetic resin material and comprises two actuating bars 35, a guide bar 36 extending between and parallel to the two actuating bars 35 and an operating portion 37 connecting the two actuating bars 35 and the guide bar 36 together on one ends thereof.
The guide bar 36 terminates in a forward broadened end portion as shown. Each of the actuating bars 35 has an actuating protrusion 38 formed on its lower surface. It is to be 25 noted that the guide bar 36 has a thickness slightly greater than the actuating bars 35.
The assembly of the tact switch is carried out by assembling the movable contact piece 28 and the actuator member 34 sequentially into the housing 11 and closing it by the top lid 24. During this operation of assembly, the movable contact piece 28 is assembled with the curved section 29 projecting downwardly convexly with the apex of thereof resting on the pedestal 14 and the positioning hole 31 fitted over the protrusion 15 for positioning the contact piece.
FIG. 3 illustrates the movable contact piece 28 assembled in the housing. FIG. 4A illustrates the actuator member 34 assembled from above onto the sub-assembly shown in FIG. 3. It is seen that the operating portion 37 of the actuator member 34 is located outside of the housing 11 with the outer portions of the forward ends of the two actuating bars 35 slightly dipped under the inclined surfaces 19b of the housing 11.
The root portions (toward the joints to the operating portion 37) of the two actuating bars 35 and the guide bar 36 are located over the notches 16 and the recess 18, respectively of the housing 11. That is, the actuating bars 35 and the guide bar 36 are inserted into the housing 11 through the notches 16 and the recess 18. The forward end of the guide bar 36 is guided in the notch 17.
In this state as shown in FIG. 4A, the actuating protrusions 38 provided on the forward ends of the actuating bars 35 lying on the marginal portions of the movable contact piece 28 in slight resilient contact therewith.
The securement of the top lid 24 to the housing 11 is effected by engaging the L-shaped latch tabs 27 with the respective engagement lugs 22. In doing so, the two engagement flaps 26 are fitted into the respective engagement recesses 23 whereby the assembly of the tact switch as shown in FIGS. 1A, 1B and 1C is completed.
With the tact switch constructed as described above, when the operating portion 37 is pushed inwardly parallel to the bottom surface of the housing 11, the forward end portions of the two actuating bars 35 are depressed downwardly toward the bottom surface of the housing 11 as they are guided by the inclined surfaces 19b. Specifically, the actuating bars 35 are flexed as shown in FIG. 5B to cause the actuating protrusions 38 to press down on the marginal portions of the movable contact piece 28.
This pressing operation causes the movable contact piece to be inverted or flipped over in its convexity-concavity orientation to thereby bring its contact portions 33 into contact with the corresponding fixed contact plates 13. This contact brings the four fixed contact plates 13 into conduction with each other to turn the switch on. It should be noted that FIG. 4B illustrated the operating portion 37 as being pushed inwardly with the top lid removed while FIG. 6 illustrates the movable contact piece 28 in its reversed state in the broken lines.
When the pushing operation on the operating portion 37 is released, the movable contact piece 28 is flipped over back to its original curved position due to its elastic restoring force to turn the switch off while at the same time the movable contact actuator member 34 is forced out of the housing 11 whereby the operating portion 37 is restored to its original position, that is, to the non-operative state shown in FIG. 5A.
It has been found out, however, that the tact switch having the construction as discussed above have some problems in attempting the automation of assembly.
Specifically, although the movable contact piece 28 and the top lid 24 may be assembled by stacking (dropping) and pushing them one on another from above, the actuator member 34 cannot be assembled simply by depositing on the underlying component from above, since it is required to dip the forward ends of the actuating bars 35 under the inclined surfaces 19b of the housing 11. In other words, the actuator member 34 need be inserted obliquely in the direction of inclination of the inclined surfaces 19b into the housing 11 for assembly. This has posed difficulties in atomizing the assembly of the switch.
In addition, it has been found that the movable contact piece 28 having the construction as described above and assembled as such in the switch has also a problem in the reliability in electrical contact with the fixed contact plates 13 in that it can cause an increase in the contact resistance and even the worst malfunction (failure to establish the switch-on state).
This is due to the configuration of the contact portions 33 provided around the marginal portions of the movable contact piece 28. More specifically, the contact portions 33 are configured to terminate in essentially linear forward ends to make line contact with the fixed contact plates 13, resulting possibly in failure to provide adequate contacting pressure. To be worse, variations in the geometry and/or assembly of the switch components can possibly cause substantial portions of linear forward ends of the contact portions 33 to lift off the fixed contact plates 13 when the witch is brought into its on-position.